1. Field of the Invention
The present invention relates to a crank angle detecting apparatus and a reference angular position detection method for determining that the crank shaft of an internal combustion engine reaches a reference angular position.
2. Description of the Related Art
When controlling the timing of fuel injection for feeding fuel to an internal combustion engine by using an injector, or when controlling the ignition timing for causing the spark plug to perform spark discharge, a rotation angular position (i.e., crank angle) of the crank shaft of the engine is detected (determined) based on the reference angular position, and then these timings are set.
A typical crank angle detecting apparatus for detecting a rotational angle of the crank shaft of the engine includes a disc-shaped rotor, which rotates with the crank shaft, and an electromagnetic pickup, which is disposed in the vicinity of the outer periphery of the rotor. A plurality of convex sections or concave sections as detection sections are provided on the outer periphery of the rotor. The convex or concave sections are made of a magnetic material, and spaced from each other at a predetermined angle. When the rotor rotates with the crank shaft, a pulse is generated from the electromagnetic pickup upon passing of the detection section in the vicinity of the electromagnetic pickup. A certain detection section is omitted at a location which corresponds to the reference angle of the crank shaft so that a special pulse is generated at the reference crank angle while the rotor rotates 360 degrees. Alternatively, a longer detection section is provided at a location which corresponds to the reference angle of the crank shaft so that a special pulse is generated. The special pulse has a different shape than other ordinary pulses. For example, the special pulse includes a relatively long no-peak period. A time point at which the crank shaft is on the reference angular position is detected, and pulses are counted on the basis of this detected time point. Then, the fuel injection timing and the ignition timing are set. Such apparatuses are disclosed in, for example, Japanese Patent Application Laid-Open (Kokai) No. S59-31406, Japanese Patent Application Laid-Open No. S59-173562, and Japanese Patent Application Laid-Open No. H6-17735.
In order to improve the accuracy of the timing setting, it is required to increase the number of detection sections on the rotor. In order to detect a time point for every fifteen degrees of the crank angle, for example, twenty four detection sections are required to be disposed at regular intervals on the outer periphery of the rotor. If the number of detection sections increases, the distance between each two detection sections becomes narrower. When the longer detection section which is elongated in the direction of rotation of the rotor should be provided on the rotor to detect the reference angular position, it is difficult in reality to form a sufficiently long detection section on the rotor surface. As a result, the accuracy of detection of the elongated detection section drops.